Each plant is housed within two of the containers. Sewage enters the first container where it is exposed to oxygen and bacteria to begin treatment. It is then pumped to a second container where it passes through a series of filters to complete the treatment process before being discharged into the Mahurangi River. By the time the water is discharged it, is very nearly clean enough to drink.

All the pumps and other equipment required to facilitate the process are included within the containers. This means once the plants are no longer required they can be picked up, transported to another site and begin treatment elsewhere. Watercare project manager Alan Brooks says one of the joys of the project is that it is so sustainable. “We have not had to waste money on constructing a building that would have to be demolished in a few years’ time,” he says.Although parts of treatment plants have been placed in containers before, it’s believed this is the first time an entire plant has been housed in this way.

The plants were designed and built by Timaru-based Apex Environmental and together they can process 250,000 litres of sewage per day.Once the plants have finished duties in Warkworth, they will likely be transferred to Waiheke Island to bolster treatment facilities there.

Apex technical director Matt Savage says building a sewage treatment plant on an island is normally a tricky and expensive process. However, all that will be required is a concrete pad to be laid and then the containers will be lifted on to them by crane. Once the system is plumbed in, it can begin operating almost immediately.

A replacement plant is due to be completed in Snells Beach in about four years. Once the Snells Beach plant is operational the Warkworth plant will be decommissioned.